As you can see the above message, whirr is trying to startup a host whose ip address is '10.245.106.99'. But, whirr hangs and doesn't startup the hadoop service. So, I tried to login to the host '10.245.106.99' via ssh.

hadoop@domU-12-31-39-00-A5-21:~$ ssh ubuntu@10.245.106.99
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@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
b1:62:ad:fd:3f:a7:29:df:7f:0c:91:ca:ed:66:8e:3a.
Please contact your system administrator.
Add correct host key in /home/hadoop/.ssh/known_hosts to get rid of this message.
Offending key in /home/hadoop/.ssh/known_hosts:8
RSA host key for 10.245.106.99 has changed and you have requested strict checking.
Host key verification failed.

I expected that whirr hangs when the file '$HOME/.ssh/known_hosts' includes the obsolete entry. Although such case may occur rarely, someone who employs many instances in EC2 would meet such case. whirr needs to prevent hanging caused by obsolete identifiers of ssh.